Magnetic resonance data acquisition systems generally comprise a large magnet for providing a static magnetic field of greater than one Tesla, for example. While other types of magnets could be used within the scope of the invention, in a preferred embodiment a superconducting magnet is used. The large static field tends to cause the average of the spins of atomic nuclei to align with the field.
Pulsed radio frequency (RF) signals are used in the presence of gradient magnetic field to nutate the aligned "spins" in selected portions of the sample. (see "Fast Scan Proton Density Imaging by NMR" by P. Mansfield et al, Jour. of Physics: Scientific Instruments 1976 Vol. 9 pp 271-278). When the RF pulse ends the nutated spins tend to return to the aligned condition. As the spins move in the magnetic field to return to alignment, free induction decay (FID) signals are generated. It is the FID signals that are most popularly used for imaging purposes.
The probes or coils used for transmitting the RF signals are also generally, though not necessarily, used for receiving the FID signals. The RF frequency used is the Larmor frequency. The Larmor frequency, as is well known, is a function of the particular element and the magnetic field strength in the nucleus' vicinity. For example, when the element is hydrogen with magnetic fields of slightly less than 2 Tesla, the Larmor frequency is greater than 80 MHz.
The static magnetic field is generated by the large magnet having a tunnel or bore into which the specimen or patient is placed. The RF coils or probes are generally built to fit within the bore and to be able to receive the patient (body probes) or parts of the patient within the coils.
A basic problem encountered with the probes and especially with the body probes is that they normally have low self-resonant frequencies. Thus, for example, the self resonance of body coils used in high magnetic fields, is generally lower than the Larmor frequency required to nutate spins in the high strength magnetic fields.
Another area of concern when using body probes in high strength magnetic fields is meeting government specifications such as the FDA regulation in the United States with regard to specific absorption rate (0.4 W/kg.) of the samples subjected to the high frequency RF pulses. A related problem is the electrostatic coupling that occurs between the probes and the subject.
The designers of the probes used in MR systems actually have at least two distinct frequencies subject to their control which they should take into account. They are the frequency of the RF power source, and the resonance frequency of the coil which is determined by its inductance and the capacitance; i.e. the length of the electrical path including distributed inductances, capacitances, and lumped inductors and capacitors.
To optimize the probe's operation the two frequencies should coincide. The RF frequency of the pulses used to nutate the spins (Larmor frequency) is determined by the element used for resonance and by the strength of the magnetic field at the spins. The body probe also should be turned to the Larmor frequency. The natural frequency of the inductors of the probe as turned by stray capacitances should be higher than the Larmor frequency. The coils of the probe should also be impedance matched to the power source so that the power supplied is utilized with a minimum of losses. In addition the probe should radiate a field whose magnitude is close to homogenous within the sample or subject.
Not surprisingly, the manufacture and supply of good RF probes for strong magnetic fields has been extremely difficult, among other reasons because of the relatively low self resonance of probes which are capable of assuring relatively homegeneous magnetic fields.
The most common design of probes used for MR data acquisition are known as saddle coils. These coils, especially when spanning a 120 deg. angle, produce a more homogenous field than do other designs. However, these saddle coils often have self resonant frequencies lower than the required Larmor frequencies. This is especially true of body probes big enough for body imaging. To overcome this inadequacy other types of coils such as "slotted cylinders" have been used in attempts to obtain higher self resonant frequencies. However, the slotted cylinder coils are not as inherently homogeneous as are the saddle coils and accordingly produce images with artifacts such as shadows which often make the images unsuitable for clinical use. Accordingly, scientists and researchers in the field are still attempting to provide probes for MR data acquisition which produce homogeneous electro-magnetic fields, minimize electrostatic coupling, provide large signal to noise ratios and have self resonant frequencies greater than 80 MHz, in fields slightly less than two Tesla.
Accordingly it is the object of the present invention to provide new and improved body probes of the type described.